CI: Remove outdated reference to ubuntu-20.04

.github/workflows/linux_builds.yml

@@ -115,7 +115,7 @@ jobs:
         run: |
           sudo rm -f /etc/apt/sources.list.d/microsoft-prod.list
           sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EB8B81E14DA65431D7504EA8F63F0F2B90935439
-          sudo add-apt-repository "deb https://ppa.launchpadcontent.net/kisak/turtle/ubuntu ${{ matrix.os == 'ubuntu-20.04' && 'focal' || 'jammy' }} main"
+          sudo add-apt-repository "deb https://ppa.launchpadcontent.net/kisak/turtle/ubuntu jammy main"
           sudo apt-get install -qq mesa-vulkan-drivers
 
       # TODO: Figure out somehow how to embed this one.